Orientation anisotropy of quantitative MRI relaxation parameters in ordered tissue

Abstract

In highly organized tissues, such as cartilage, tendons and white matter, several quantitative MRI parameters exhibit dependence on the orientation of the tissue constituents with respect to the main imaging magnetic field (B₀). In this study, we investigated the dependence of multiple relaxation parameters on the orientation of articular cartilage specimens in the B₀. Bovine patellar cartilage-bone samples (n = 4) were investigated ex vivo at 9.4 Tesla at seven different orientations, and the MRI results were compared with polarized light microscopy findings on specimen structure. Dependences of T₂ and continuous wave (CW)-T_1ρ relaxation times on cartilage orientation were confirmed. T₂ (and T₂*) had the highest sensitivity to orientation, followed by T_RAFF2 and adiabatic T_2ρ. The highest dependence was seen in the highly organized deep cartilage and the smallest in the least organized transitional layer. Increasing spin-lock amplitude decreased the orientation dependence of CW-T_1ρ. T₁ was found practically orientation-independent and was closely followed by adiabatic T_1ρ. The results suggest that T₁ and adiabatic T_1ρ should be preferred for orientation-independent quantitative assessment of organized tissues such as articular cartilage. On the other hand, based on the literature, parameters with higher orientation anisotropy appear to be more sensitive to degenerative changes in cartilage.

Figure 1

PLM results presented as a map for one representative sample and profiles of all the specimens: (a) orientation; (b) retardation; (c) anisotropy. Articular surface and cartilage-bone interface are marked on the maps with arrowheads. Profiles were calculated separately for each sample along cartilage depth and averaged point-by-point for mean profile.

Figure 2

Relaxation parameter maps for one representative sample at different angles with respect to B₀ (arrows above). Orientation anisotropy is clearly seen for T₂, T₂*, Ad-T_2ρ and T_RAFF2. Articular surface and cartilage-bone interface are marked with arrowheads.

Figure 3

Interpolated profile maps for relaxation parameters (one representative sample). Articular surface is on the left, bone interface towards right, with the profiles at different orientations stacked on top of each other and then interpolated. SZ, TZ and RZ denote the approximate locations of the histological zones.

Figure 4

Depth-wise MR anisotropy profiles for relaxation parameters (average of four samples). Smallest anisotropy is noted at the location of the transitional zone, while the maximum anisotropy is observed in the deep zone. Boxed area shows the depth used for calculating deep cartilage average values.

Figure 5

Average anisotropy of relaxation parameters in deep cartilage versus relative difference of relaxation parameters between “normal” and “OA” in different models as reported in literature (o = human in vivo ^{, ,} , □ = human ex vivo , ∆ = in vivo animal OA model with imaging done for samples, ◊ = ex vivo animal model). Shaded area represents deviation in anisotropy and range of relative difference values. For T₂*, Adiabatic T_1ρ with HS8 pulses, and CW-T_1ρ 2000 Hz no reference values were found. The optimal parameter would lie in the lower right corner.